Diagnosting Problems with D/C Board
SHARE |
|
|
by Bill Degnan - 12/16/2014 19:59 |
 Using a #8 deep socket I removed the 9 bolts that hold the front panel cover from the underlying MITS Altair 8800b Front Panel D/C Board. Click image for larger view.
I downloaded the hardware manual for the 8800b to look up the expected voltage values. This is a good way to track down electrical problems - bad voltages often indicate a bad part in circuit. The first things to test are the components near the two voltage regulators on the D/C board, sections V1 and V2.  Next I started testing the caps and voltage regulator in the VR2 (pictured above) and VR1 areas. Pictured is VR2, click image for larger view.
 I noticed right away, even after applying power for only a few seconds that the resistor in the back of the board connected to R76 / R75 was cooking. Note the burned Styrofoam insulation. The resistor is supposed to be 5W / 5ohm but it measured around 7.7ohms. This is too high to be in spec so I removed it and ordered a replacement. Testing the voltage regulators more carefully than I was able to before showed they were still basically OK. Voltages to the S-100 bus were also OK. Click image for larger view.
I am not saying the only issue is the 5W 5ohm resistor in V2, but this is certainly not quite right. The smart thing to do in general with old MITS boards is replace all the vulnerable caps. I will start with VR2 and VR1. I also want to test all of the TTL chips. Fortunately they're all socketed and I have a TTL tester to make this an easy job. After the apparently bad resistor, caps and any bad TTL chips are replaced I will run tests via the checkout procedure using a logic probe and volt meter. Also an oscilloscope if needed. Or maybe the front panel will begin to work correctly and that will be that...stay tuned. Reply |
|
Resources:
Popular Topics and FAQs
Past Issues:
ATT UNIX PC 3B1 370429065 c
This image was selected at random from the archive. Click image for more photos and files from this set.